Method for producing coating powders catalysts and drier water-borne coatings by spraying compositions with compressed fluids

ABSTRACT

This invention relates to methods for spraying liquid compositions containing volatile solvent by using compressed fluids, such as carbon dioxide or ethane, to form solid particulates, coating powders, and catalyst materials, which can be produced with narrow particle size distributions and can be sprayed at higher solids levels, in ambient air or with heated air applied to just the spray instead of a spray chamber. Novel catalyst supports can be produced having a beneficial morphology such as for olefin catalysis. Drier water-borne coatings can be applied to substrates by using compressed fluids to spray water-borne coating compositions having conventional water levels, thereby reducing runs and sags and shortening dry times.

BRIEF SUMMARY OF THE INVENTION

1. Technical Field

This invention relates to spraying liquid compositions with solventevaporation in order to produce drier compositions such as particulatesand coating films. More particularly, this invention relates to methodsfor spraying liquid compositions containing volatile solvent by usingcompressed fluids, such as carbon dioxide or ethane, to form solidparticulates, coating powders, and catalyst materials and to apply drierwater-borne coatings from water-borne coating compositions havingconventional water levels.

2. Background of the Invention

Improved methods are needed by which materials such as particulates,coating powders, and catalyst materials can be produced by sprayingwithout requiring high energy use like that used in conventional spraydrying. Methods are needed by which such materials can be produced athigher solids levels and without using hot gas, or by supplying arelatively small amount of heated gas locally to just the spray, insteadof heating an entire spray chamber. Such methods would also enabletemperature sensitive materials to be spray dried at lower temperatureor at essentially ambient temperature. Furthermore, an improved methodof producing such particulate materials is desirable wherein the powdersproduced have a narrow particle size distribution, which often improvesthe performance of powders in applications. For example, it is desirablefor a coating powder to have minimal large particles which give poorcoating appearance and minimal small particles which become oversprayand waste and which build up to an unacceptable level in recycledpowder. Furthermore, such a spray method of producing coating powderswould be desirable as an alternative to costly milling or cryogenicgrinding. Similarly, it is desirable for catalysts used in fluidized bedreactors, such as in polyethylene production, to have a narrow dropletsize distribution for efficient use of the catalyst and to give moreuniform pellets and better performance. An improved method is alsoneeded by which water-borne coatings having conventional water levelscan be sprayed but with drier coating films applied, in order to improvecoating performance and shorten dry times.

DISCLOSURE OF THE INVENTION

Particulates, coating powders, and catalyst materials can be produced byspraying at higher solids levels and at lower temperature, without usinghot gas or by supplying heated gas to the spray instead of an entirespray chamber. Furthermore, they can be produced with relatively narrowparticle size distributions. Catalyst particles can also be producedhaving a novel, beneficial morphology. Water-borne coatings withconventional water levels can be sprayed with drier coating filmsapplied, thereby improving their performance and shortening dry times.

In one embodiment, this invention relates to a process for forming solidparticulates by spraying a liquid solvent-borne composition, whichcomprises:

(1) forming a liquid mixture in a closed system, said liquid mixturecomprising:

(a) a solvent-borne composition comprising:

(i) a nonvolatile materials fraction which is solid or capable ofbecoming solid, which is capable of being sprayed, and which is capableof forming solid particulates by solvent evaporation when sprayed in(2); and

(ii) a solvent fraction which is sufficiently volatile to render saidsolvent-borne composition capable of forming solid particulates whensprayed in (2); and

(b) at least one compressed fluid in an amount which when added to (a)renders said liquid mixture capable of forming a substantiallydecompressive spray in (2), wherein the compressed fluid is a gas atstandard conditions of 0° Celsius and one atmosphere pressure (STP); and

(2) spraying said liquid mixture at a temperature and pressure thatgives a substantially decompressive spray by passing the mixture throughan orifice into an environment suitable for forming solid particulatesby solvent evaporation, wherein the spray has an average particle sizegreater than about one micron.

In a preferred embodiment, the at least one compressed fluid is selectedfrom the group consisting of carbon dioxide, nitrous oxide, ethane,ethylene, propane, and propylene. The most preferred compressed fluidsare carbon dioxide and ethane. The compressed fluid is preferably asupercritical fluid at the temperature and pressure at which the liquidmixture is sprayed. The liquid mixture is preferably heated to atemperature that substantially compensates for the drop in spraytemperature that occurs due to expansion cooling of the decompressingcompressed fluid, in order to increase the evaporation rate of solventfrom the spray.

In another preferred embodiment, the solvent fraction of the solventborne composition has an average relative evaporation rate greater thanabout 70.

In still another preferred embodiment, the solid particulates thusformed have a narrow particle size distribution.

In yet another preferred embodiment, at least one gas flow is applied tothe substantially decompressive spray to increase the rate of turbulentmixing or the temperature within the spray or both.

In another embodiment, this invention relates to a process for forming acoating-powder by spray drying a liquid precursor coating-powdercomposition, which comprises:

(1) forming a liquid mixture in a closed system, said liquid mixturecomprising:

(a) a precursor coating-powder composition comprising:

(i) a solids fraction containing dry ingredients of a coating-powder andwhich is capable of forming powder by solvent evaporation when sprayedin (2); and

(ii) a solvent fraction which is at least partially miscible with (i)and which is sufficiently volatile to render said precursorcoating-powder composition capable of forming powder when sprayed in(2); and

(b) at least one compressed fluid in an amount which when added to (a)renders said liquid mixture capable of forming a substantiallydecompressive spray in (2), wherein the compressed fluid is a gas atstandard conditions of 0° Celsius and one atmosphere pressure (STP); and

(2) spraying said liquid mixture at a temperature and pressure thatgives a substantially decompressive spray by passing the mixture throughan orifice into an environment suitable for forming coating powder bysolvent evaporation.

In a preferred embodiment, the at least one compressed fluid is carbondioxide or ethane and is a supercritical fluid at the temperature andpressure at which said liquid mixture is sprayed.

In another preferred embodiment, the coating-powder formed has a narrowparticle size distribution with a span of less than about 2.0.

In still another preferred embodiment, the coating-powder contains atleast one polymer selected from the group consisting of epoxies,polyesters, acrylics, polyurethanes, epoxy-polyester hybrids, blockedisocyanates, cellulosics, vinyls, polyamides, and hybrid polymersthereof.

In yet another preferred embodiment, the process may further comprisedepositing said coating-powder on to a substrate and heating thesubstrate to form a coating film.

In another preferred embodiment, at least one gas flow is applied to thesubstantially decompressive spray to increase the rate of turbulentmixing or the temperature within the spray or both.

In still another embodiment, this invention relates to a process forforming a catalyst, catalyst support, or catalyst precursor by spraydrying a liquid precursor catalyst composition, which comprises:

(1) forming a liquid mixture in a closed system, said liquid mixturecomprising:

(a) a precursor catalyst composition comprising:

(i) a solids fraction containing dry ingredients of a catalyst, catalystsupport, or catalyst precursor and which is capable of formingparticulates by solvent evaporation when sprayed in (2); and

(ii) a solvent fraction which is at least partially miscible with (i)and which is sufficiently volatile to render said precursor catalystcomposition capable of forming particulates when sprayed in (2); and

(b) at least one compressed fluid in an mount which when added to (a)renders said liquid mixture capable of forming a substantiallydecompressive spray in (2), wherein the compressed fluid is a gas atstandard conditions of 0° Celsius and one atmosphere pressure (STP); and

(2) spraying said liquid mixture at a temperature and pressure thatgives a substantially decompressive spray by passing the mixture throughan orifice into an environment suitable for forming particulates bysolvent evaporation.

In a preferred embodiment, the solids fraction contains at least oneorganic polymer and at least one inorganic or organometallic compound,and the catalyst, catalyst support, or catalyst precursor particulateformed comprises an aggregate of solid microparticulates containing saidat least one inorganic or organometallic compound which are at leastpartially enclosed in a polymeric shell.

In yet another embodiment, this invention relates to a process forforming solid particulates by spray drying a liquid water-bornecomposition, which comprises:

(1) forming a liquid mixture in a closed system, said liquid mixturecomprising:

(a) a water-borne composition comprising:

(i) a nonvolatile materials fraction which is solid or capable ofbecoming solid, which is capable of being sprayed, and which is capableof forming solid particulates by evaporation when sprayed in (2); and

(ii) a solvent fraction containing at least water; which is sufficientlyvolatile to render, and contains water in an amount which renders, saidwater-borne composition capable of forming solid particulates whensprayed in (2); and

(b) at least one compressed fluid which is a supercritical fluid at thetemperature and pressure at which said liquid mixture is sprayed andwhich is substantially present in said liquid mixture as a finelydispersed liquid compressed fluid phase, in an amount which renders saidliquid mixture capable of forming a substantially decompressive spray in(2), wherein the compressed fluid is a gas at standard conditions of 0°Celsius and one atmosphere pressure (STP); and

(2) spraying said liquid mixture at a temperature above about 40°Celsius and a pressure that gives a substantially decompressive spray bypassing the mixture through an orifice into an environment suitable forforming solid particulates by evaporation.

This invention also relates to a process for applying a water-bornecoating to a substrate which comprises:

(1) forming a liquid mixture in a closed system, said liquid mixturecomprising:

(a) a water-borne coating composition containing a water level whichrenders the liquid mixture capable of being sprayed conventionally withno compressed fluid; which is capable of forming a coating on asubstrate; and which contains a solvent fraction having at least about35 percent water by weight; and

(b) at least one compressed fluid which is substantially present in saidliquid mixture as a finely dispersed liquid compressed fluid phase; andwhich is in an amount which renders said liquid mixture capable offorming a substantially decompressive spray, wherein the compressedfluid is a gas at standard conditions of 0° C. and one atmospherepressure (STP); and

(2) spraying said liquid mixture at a temperature and pressure thatgives a substantially decompressive spray by passing the mixture throughan orifice into an environment suitable for water evaporation andapplying a coating to a substrate.

In a preferred embodiment, the water-borne coating composition containsat least one polymer which is a water-dispersible polymer or awater-soluble polymer.

In another preferred embodiment, the compressed fluid is carbon dioxideand the pH of said liquid mixture is controlled to prevent polymerprecipitation when the carbon dioxide is admixed with said water-bonecoating composition.

In still another preferred embodiment, the at least one compressed fluidis carbon dioxide or ethane and is a supercritical fluid at thetemperature and pressure at which said liquid mixture is sprayed, andsaid water-borne coating composition contains at least one organicsolvent that is capable of being extracted from said water-borne coatingcomposition into the compressed fluid, thereby enabling said compressedfluid to form the liquid compressed fluid phase at the supercriticaltemperature and pressure.

In yet another preferred embodiment, the at least one compressed fluidis carbon dioxide or ethane and is a supercritical fluid at thetemperature and pressure at which said liquid mixture is sprayed, andsaid liquid mixture contains in addition at least one organic solvent(c) which is immiscible with said water-borne coating composition; whichis at least partially miscible with said at least one compressed fluidunder pressure; and which is present at least in an amount which enablessaid compressed fluid to form the liquid compressed fluid phase at thesupercritical temperature and pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a narrow particle size distribution having a span of 1.3 andaverage particle size of 21 microns produced by spraying a solution of asolid acrylic polymer, volatile solvent, and compressed carbon dioxideas a decompressive spray.

FIG. 2 compares particle size distributions for catalyst supportsproduced by using compressed fluid (A) and conventional thermal spraydrying (B).

DETAILED DESCRIPTION

It has been found that, by using the methods of this invention, liquidsolvent-borne compositions can be sprayed with compressed fluids such ascarbon dioxide and ethane to form solid particulates, coating powders,catalyst materials, and the like, by solvent evaporation at mildconditions without using large amounts of energy as in conventionalspray drying. Furthermore, particulates and powders can be formed havingrelatively narrow particle size distributions. Liquid water-bonecompositions and water-borne coatings with conventional water levels,for which compressed fluids such as carbon dioxide and ethane have verylow solubility, can be sprayed by using a finely dispersed liquidcompressed fluid phase in the composition or coating, to form solidparticulates or to apply drier coatings with fine atomization to givegood coating appearance and performance and shorter dry times.

It has been discovered that a substantially decompressive spray,produced by using at least one compressed fluid in a sufficiently highamount and at a suitable spray temperature and pressure, can produce ahigh rate of solvent evaporation from the spray if the solvent orsolvent blend is sufficiently volatile, and thereby produce solidparticulates, powders, or drier water-borne coating films fromwater-borne coating compositions having conventional water levels thatenable them to be sprayed by conventional spray methods.

It has been discovered that decompressive sprays can indeed produceenhanced evaporation of solvent provided that the solvent or solventprofile has a sufficiently high average relative evaporation rate, eventhough very little evaporation occurs for the slow evaporating solventsused in coating concentrates. Without wishing to be bound by theory, itis believed that the high evaporation rate is caused by an exceptionallyhigh mass transfer rate that occurs during formation of thedecompressive spray due to the extremely rapid gasification of thedissolved compressed fluid, which overcomes effects of the rapidtemperature drop that suppresses volatility. The fast and mediumevaporating solvents are much more affected by these intense masstransfer conditions than the slow evaporating solvents. Furthermore, ithas been discovered that the expansion of the decompressing gas thatgenerates the decompressive spray can overcome the higher viscositygenerated during the atomization process by the greater evaporation ofthe faster evaporating solvents than occurs in coating application.Therefore fine atomization can result. Indeed, if the viscosity becomestoo high, then the decompressive spray is incapable of forming, but thebelief that slow evaporating solvent is required in order to maintainsufficient fluidity during atomization has been found not to be true.

As used herein, it will be understood that a "compressed fluid" is afluid which may be in its gaseous state, its liquid state, or acombination thereof, or is a supercritical fluid, depending upon (i) theparticular temperature and pressure to which it is subjected, (ii) thevapor pressure of the fluid at that particular temperature, and (iii)the critical temperature and critical pressure of the fluid, but whichis in its gaseous state at standard conditions of 0° Celsius temperatureand one atmosphere absolute pressure (STP). As used herein, a"supercritical fluid" is a fluid that is at a temperature and pressuresuch that it is at, above, or slightly below its critical point.

Compounds which may be used as compressed fluids in the presentinvention include but are not limited to carbon dioxide, nitrous oxide,ammonia, xenon, ethane, ethylene, propane, propylene, butane, isobutane,chlorotrifluoromethane, monofluoromethane, and mixtures thereof.Preferably, the compressed fluid is or can be made environmentallycompatible or can be readily recovered from the spray environment. Theutility of any of the above-mentioned compressed fluids in the practiceof the present invention will depend upon the composition used, thetemperature and pressure of application, and the inertness and stabilityof the compressed fluid.

In general, carbon dioxide, nitrous oxide, ethane, ethylene, propane,and propylene are preferred compressed fluids in the present invention.However, nitrous oxide should be used only under safe and stableconditions. Due to environmental compatibility, low toxicity, and highsolubility, carbon dioxide and ethane are more preferred compressedfluids. Due to low cost, non-flammability, and wide availability, carbondioxide is generally the most preferred compressed fluid. However, useof any of the aforementioned compounds and mixtures thereof are to beconsidered within the scope of the present invention.

As used herein, the phrases "solvent-borne composition", "coating-powdercomposition", "precursor catalyst composition", "water-bornecomposition", "water-borne coating composition", "coating composition","coating formulation", and "coating material" are understood to meancompositions, formulations, and materials that have no compressed fluidadmixed therewith.

As used herein, the term "solvent" is understood to mean conventionalsolvents that have no compressed fluid admixed therewith and which arein the liquid state at conditions of about 25° C. temperature and oneatmosphere absolute pressure.

The liquid solvent-borne compositions that may be used with thisinvention are generally comprised of 1) a nonvolatile materials fractionwhich is solid or capable of becoming solid, which is capable of beingsprayed, and which is capable of forming solid particulates by solventevaporation when sprayed as a decompressive spray; and 2) a solventfraction which is sufficiently volatile to render said solvent-bornecomposition capable of forming solid particulates when sprayed as adecompressive spray.

In general, the nonvolatile materials fraction is the fraction of thesolvent-borne composition that remains after the solvent fraction hasevaporated and therefore it is the fraction that forms the solidparticulates. The nonvolatile materials fraction comprises polymers,resins, waxes, organic compounds, inorganic compounds, and othernonvolatile materials that are solid or a capable of becoming solidduring spraying, such as rapidly reacting two-component polymer systemsthat are mixed as they are sprayed to initiate the rapid reaction.Dilution or blocking by the dissolved compressed fluid may retard thereaction until the mixture is sprayed. Examples of particulates that maybe formed include plastics, resins, detergents, pesticides, pigments,dyestuffs, organic chemicals, and inorganic chemicals. The nonvolatilematerials fraction may be sprayed as a solution, emulsion, dispersion,or suspension in the solvent fraction. In general, divided solids thatare dispersed should have particle sizes that are sufficiently small tomaintain a dispersed state and to pass readily through the sprayorifice. Divided solids with particle sizes too large to maintain astable dispersion may be used if a dispersion or suspension can beformed and maintained by agitation. Preferably, the nonvolatilematerials fraction contains dispersed solids that have an averageparticle size less than about 25 microns and more preferably less thanabout 10 microns.

Solid polymers may be dissolved or dispersed, but generally they are atleast partially miscible with the solvent fraction. Solid polymersshould have sufficiently high molecular weight and a sufficiently highglass transition temperature to form solid particulates by solventevaporation. The glass transition temperature should be above about 25°C., preferably above 30° C., more preferably above 40° C., and mostpreferably above 50° C. Suitable polymers include but are not limited toacrylics, polyesters, cellulosics, polyolefins, epoxies, alkyds, vinyls,polyurethanes, silicone polymers, rubbers, and thermoplastic polymers ingeneral, and mixtures thereof.

The nonvolatile materials fraction must be a sufficiently high fractionof the solvent-bone composition to be capable of forming solidparticulates by solvent evaporation when sprayed by the decompressivespray and to form particulates of surf dent size. The fraction requiredwill generally depend upon the volatility of the solvent fraction, witha higher fraction being required for lower volatility. The nonvolatilematerials fraction should generally be greater than about 10% by weightof the solvent-borne composition, preferably greater than about 15%,more preferably greater than about 20%, and most preferably greater thanabout 25%. Generally a higher fraction is desirable so as to lessen theamount of solvent that must be evaporated. However, the nonvolatilematerials fraction must not be so excessively high a fraction that itrenders the solvent-borne composition unable to form a substantiallydecompressive spray or to form a suitable particulate size. The suitableupper limit will depend upon the physical and chemical characteristicsof the particular nonvolatile materials fraction, such as the molecularweight of polymers, the degree of intermolecular cohesion, the mount andnature of dispersed solids, reactivity, and the like. Generally lowermolecular weight and less cohesive materials can be sprayed at highernonvolatile materials levels because they remain more fluid at thehigher levels. The nonvolatile materials fraction should generally beless than about 90% by weight of the solvent-borne composition,preferably less than about 80%, more preferably less than about 70%, andmost preferably less than about 60%.

The viscosity of solvent-borne compositions that are capable of forminga decompressive spray has proven to be an insensitive correlatingparameter for sprayability. Solvent-borne compositions have been finelyatomized with viscosities that range from below 100 to above 20,000centipoise. However, the solvent-borne composition will generally have aviscosity of from about 500 to about 5000 centipoise, preferably fromabout 800 to about 3000 centipoise, as measured at a temperature ofabout 25° C.

The nonvolatile materials fraction must be capable of forming solidparticulates by solvent evaporation when sprayed by the decompressivespray. The nonvolatile materials fraction preferably should retain asufficiently "open" structure as the solid particulates are formed tofacilitate diffusion, transport, and evaporation of solvent from theinterior of the particulates.

In addition to the nonvolatile materials fraction, a solvent fraction isalso employed which is sufficiently volatile to render saidsolvent-borne composition capable of forming solid particulates whensprayed by the decompressive spray. The solvent may perform a variety offunctions, such as to dissolve polymers and other nonvolatile materials,to reduce viscosity, to provide a carrier medium for dispersions, andthe like. Generally the solvent fraction is at least partially misciblewith the nonvolatile materials fraction. Polymeric compositionsgenerally contain at least one active solvent for the polymer. Theselection of a particular solvent fraction for a given nonvolatilematerials fraction to obtain desired solubility and dispersibilitycharacteristics is well known to those skilled in the art.

Based on a relative evaporation rate (RER) to a butyl acetate standardequal to 100 using ASTM Method D3599 at 25° C. and one atmospherepressure, to be sufficiently volatile, the solvent fraction desirablyhas an average relative evaporation rate greater than about 70, wherethe average relative evaporation rate of a mixture of solvents iscalculated as the inverse weighted average of the individual solventrelative evaporation rates, that is, 1/RER_(AVG) =M_(1/) RER₁ +M_(2/)RER₂ +M_(3/) RER₃ +. . . , where M_(i) are the weight fractions of theindividual solvents. The average relative evaporation rate is preferablygreater than about 85, more preferably greater than about 105, stillmore preferably greater than about 140, and most preferably greater thanabout 175. The average relative evaporation rate is preferably less thanabout 4000, more preferably less than about 3000, and most preferablyless than about 2000.

In general, the solvent fraction preferably contains less than about 10%by weight of solvents with relative evaporation rates below about 20,more preferably less than 5%, and most preferably less than 2%. Inaddition, the solvent fraction preferably contains less than about 5% byweight of solvents with relative evaporation rates below about 10, morepreferably less than about 2%, and most preferably about 0%.

Solvents comprising the solvent fraction should be sufficiently fastevaporating to give a sufficiently high average relative evaporationrate. Suitable solvents include but are not limited to ketones such asacetone, methyl ethyl ketone, methyl propyl ketone, methyl isobutylketone, methyl butyl ketone, and other aliphatic ketones; esters such asmethyl acetate, ethyl acetate, isopropyl acetate, n-propyl acetate,isobutyl acetate, butyl acetate, ethyl propionate, and other alkylcarboxylic esters; ethers such as isopropyl ether, tetrahydrofuran,ethyl butyl ether, ethyl isopropyl ether, and other aliphatic ethers;volatile glycol ethers such as ethylene glycol dimethyl ether, ethyleneglycol diethyl ether, and propylene glycol monomethyl ether; alcoholssuch as methanol, ethanol, propanol, isopropanol, isobutanol, and otheraliphatic alcohols; hydrocarbons such as hexane, toluene, Varnish Makersand Painters (VM&P) naptha, octane, 3-methyl heptane, 2,2-dimethylhexane, and other aliphatics; and nitroalkanes such as nitroethane andnitropropane.

For spraying, the solvent-borne composition is first admixed with atleast one compressed fluid to form a liquid mixture in a closed system,the compressed fluid being in an amount which renders the liquid mixturecapable of forming a substantially decompressive spray. The liquidmixture is then sprayed at a temperature and pressure that gives asubstantially decompressive spray by passing the mixture through anorifice into an environment suitable for forming solid particulates bysolvent evaporation. Substantially decompressive sprays generally formwithin a relatively narrow range of combinations of compressed fluidconcentration and spray temperature and pressure, which varies with thecharacteristics of the particular solvent-borne composition. Importantcharacteristics are the composition and amount of the nonvolatilematerials fraction, the composition of the solvent fraction, and thecomposition of the compressed fluid used. Therefore, the conditionssuitable for forming the substantially decompressive spray generallymust be determined experimentally for a given spray mixture and spraytip. However, the decompressive spray region typically follows thesolubility limit of the compressed fluid in the solvent-bornecomposition as it changes with temperature and pressure, as disclosed incopending U.S. patent application Ser. No. 129,256, filed Sep. 29, 1993(now U.S. Pat. No. 5,464,154). At constant pressure, the solubilitydecreases at higher temperature. The solubility increases with higherpressure. The decompressive spray region generally occurs at acompressed fluid concentration that is somewhat below the solubilitylimit, often being within about five weight percentage points of it orless. Frequently spraying is done at the solubility limit, or just belowor above it. A sufficiently high spray pressure is used to obtain asufficiently high solubility. The spray temperature and compressed fluidconcentration are then adjusted to give a decompressive spray having thedesired characteristics for a particular application, such as desiredparticle size. The solubility will also change with the compressed fluidused; carbon dioxide generally has significantly higher solubility thanethane. The solubility will also change with the level of nonvolatilematerials fraction, being lower for a higher solids content. Atcompressed fluid concentrations above the solubility limit, at higherpressures the liquid mixture generally comprises a liquid nonvolatilematerials phase and a liquid compressed fluid phase containing extractedsolvent, whereas at lower pressures the excess compressed fluid forms agaseous phase.

In general, the amount of compressed fluid used will be at least about5% by weight, based upon the total weight of compressed fluid andsolvent-borne composition, preferably at least about 10%, depending uponthe solubility. For carbon dioxide being the compressed fluid, due toits generally higher solubility, the amount used more preferably will beat least about 15%, still more preferably at least about 20%, and mostpreferably at least about 25% and will exceed the minimum level requiredto obtain a substantially decompressive spray. The amount of compressedfluid may exceed the solubility limit if desired, but it should not beso excessively high that the excess compressed fluid phase undulyinterferes with spray formation, such as by not remaining well dispersedin the liquid mixture or giving poor atomization. Using excesscompressed fluid can sometimes be advantageous if the pressure is highenough for a liquid excess compressed fluid phase to form which extractssolvent from the liquid solvent-borne composition. Then less solventmust evaporate from the solvent-borne composition when sprayed. However,the solvent loss increases the viscosity, so the amount extracted shouldnot be so great as to interfere with spray formation or causeundesirable spray characteristics, such as an overly large averageparticle size. If desired, excess compressed fluid can be used toseparate a portion of the solvent from the spray mixture prior tospraying, by using the methods disclosed in U.S. Pat. No. 5,290,604.Generally the liquid mixture will contain less than about 60% compressedfluid by weight.

Although high spray pressures of 5000 psig and higher may be used,preferably the spray pressure of the liquid mixture is below about 3000psig, more preferably below about 2500 psig. Very low pressure isgenerally not compatible with high compressed fluid solubility in thesolvent-borne composition. Therefore, preferably the spray pressure isabove about 50% of the critical pressure of the compressed fluid, morepreferably above about 75% of the critical pressure, still morepreferably above the critical pressure, and most preferably above about125% of the critical pressure to give higher solubility at highertemperature. For carbon dioxide being the compressed fluid, preferablythe spray pressure is above about 500 psig, more preferably above about800 psig, still more preferably above about 1100 psig, and mostpreferably above about 1400 psig.

Preferably, the spray temperature of the liquid mixture is below about150° C., more preferably below about 100° C., and most preferably belowabout 80° C. The temperature level that may be utilized will in generaldepend upon the characteristics of the solvent-borne composition, suchas stability and heat sensitivity. Reactive systems must generally besprayed at lower temperature than non-reactive systems. Preferably, thespray temperature of the liquid mixture is above about 25° C., morepreferably above about 30° C., still more preferably above about 40° C.,and most preferably above about 50° C. In order to increase theevaporation rate of solvent from the spray, the liquid mixture ispreferably heated to a temperature that substantially compensates forthe drop in spray temperature that occurs due to expansion cooling ofthe decompressing compressed fluid. Substantially decompressive sprayscan generally be formed over a range of temperatures by varying theamount of compressed fluid accordingly as the solubility varies. Inorder to evaporate solvent more rapidly from the spray to form solidparticulates, it is desirable for a higher spray temperature to be used.The spray temperature should be sufficiently high to providesufficiently fast solvent evaporation for the average relativeevaporation rate of the solvent fraction used and the amount of solventthat must be evaporated. Generally a higher spray temperature ispreferred for a lower average relative evaporation rate.

Preferably the at least one compressed fluid is a supercritical fluid atthe temperature and pressure at which the liquid mixture is sprayed.

To spray polymeric solvent-borne compositions with enhanced atomization,the liquid mixture desirably contains the at least one compressed fluidin an amount that enables the liquid mixture to form a liquid compressedfluid phase at the spray temperature, and the spray pressure desirablyis above the minimum pressure at which the liquid mixture forms a liquidcompressed fluid phase at the spray temperature, as disclosed by themethods in U.S. Pat. No. 5,290,603. Preferably, the spray pressure isabove or just below the maximum pressure at which the liquid mixtureforms a liquid compressed fluid phase at the spray temperature. Withoutwishing to be bound by theory, enhanced atomization is believed toresult because the dissolved compressed fluid, during depressurizationin the spray orifice, nucleates to form a liquid compressed fluid phasebefore forming gaseous compressed fluid, instead of nucleating directlyto a gaseous compressed fluid phase. Therefore, nucleation occurs morequickly, so gasification of the compressed fluid is more intense.

An orifice is a hole or an opening in a wall or housing, such as in aspray tip. Spray orifices, spray tips, spray nozzles, and spray gunsused for conventional and electrostatic airless and air-assisted airlessspraying of coating formulations are generally suitable for spraying theliquid mixtures of the present invention. Spray guns, nozzles, and tipsare preferred 1) that do not have excessive flow volume between theorifice and the valve that turns the spray on and off and 2) that do notobstruct the wide angle at which the spray typically exits the sprayorifice. The most preferred spray tips and spray guns are the UNICARB®spray tips and spray guns manufactured by Nordson Corporation. Orificesizes of from about 0.007-inch to about 0.0254inch nominal diameter arepreferred, although smaller and larger orifice sizes may be used.Devices and flow designs, such as pre-orifices or turbulence promoters,that promote turbulent or agitated flow in the liquid mixture prior topassing the mixture through the orifice may also be used. Thepre-orifice preferably does not create an excessively large pressuredrop in the flow of liquid mixture. The spray pattern may be a circularspray such as is produced from a round orifice or it may be an oval orflat spray as produced by a groove cut through the orifice, asaforementioned. A wider, flat spray is favorable for mixing the ambientgas of the spray environment more rapidly into the interior of thedecompressive spray, and therefore for increasing evaporation rate.However, for particularly viscous solvent-bone compositions or for highrelative evaporation rates, a more oval or circular spray may bedesirable to minimize polymer buildup on the spray tip. A favored spraytip design has two intersecting grooves cut through the orifice outletat right angles to each other. This produces two intersecting spray fanswhich produce a more axisymmetric spray pattern but give better mixingof ambient gas into the spray interior than a circular spray.

A decompressive spray can generally be obtained at lower compressedfluid concentrations and lower temperatures, than are obtained withconventional short airless spray orifices, by using an elongated orificepassageway, as disclosed in copending U.S. patent application Ser. No.061,822, filed May 13, 1993. Without wishing to be bound by theory, itis believed that the elongated orifice passageway increases the timeavailable for nucleation to a gaseous compressed fluid phase to occur,for the compressed fluid concentration, temperature, and pressure atwhich the liquid mixture is sprayed. Preferably, the ratio of length todiameter is greater than about 2 and less than about 20, more preferablygreater than about 3 and less than about 15, most preferably greaterthan about 4 and less than about 10. So too, the length of the orificepassageway should desirably be in the range of from about 0.020 inch toabout 0.400 inch, and more preferably from about 0.040 inch to about0.300 inch.

The decompressive spray is formed by passing the liquid mixture throughan orifice into a gaseous environment suitable for forming solidparticulates by solvent evaporation. The environment in which thepresent invention is conducted is not narrowly critical. However, thepressure therein must be substantially lower than the spray pressure inorder to obtain sufficient decompression of the compressed fluid to formthe decompressive spray. Preferably, the gaseous environment is at ornear atmospheric pressure. The environment will generally comprise air,but other gaseous environments may also be used, such as air withreduced oxygen content or inert gases such as nitrogen, carbon dioxide,helium, argon, or xenon, or a mixture. Oxygen or oxygen enriched air isnot desirable, because oxygen enhances the flammability of organicmaterials. The gaseous environment should contain sufficiently lowpartial pressures of the solvents contained in the solvent-bornecomposition in order to promote sufficiently rapid evaporation of thesolvent from the spray. Very low partial pressures are preferred. Thepartial pressures of the solvents should be maintained significantlybelow the point at which a fire or explosion hazard would exist in thespray environment.

The rate of solvent evaporation from the spray can be increased byapplying at least one gas flow to the substantially decompressive sprayto increase the rate of turbulent mixing or the temperature within thespray or both. A higher level of turbulent mixing increases the amountof drier external gas or air that is brought into the spray interior,which lowers the partial pressures of the solvents. Increasing thetemperature within the spray increases the vapor pressure of thesolvents.

To increase turbulent mixing, the at least one gas flow may comprise atleast one gaseous jet applied to the spray, such as compressed gas jetsused to assist atomization in air-assisted airless sprays or to modifythe shape of the spray pattern. The assist gas is typically compressedair at pressures from about 10 to about 80 psig, with pressures of about20 to about 60 psig preferred, but may also be air with reduced oxygencontent or inert gases such as compressed nitrogen, carbon dioxide, or amixture. Assist gas jets typically have little or no effect on theatomization of a decompressive spray. The gas flow may also be suppliedby one or more auxiliary tubes feeding compressed gas or air that arepositioned to discharge into the decompressive spray so as to increasethe rate of turbulent mixing. Other methods may also be used.

The gas flow or assist gas jets may be heated to increase thetemperature of the spray, such as to counteract the cooling effect ofthe compressed fluid. Although higher temperatures may be used, thetemperature of the heated gas or air flow preferably is from about 30°C. to about 90° C., more preferably from about 50° C. to about 70° C.Higher temperature increases solvent volatility, but at a constantcompressed gas pressure, higher temperature decreases the density of thegas and hence the mass flow rate, which can lower the mixing intensity.The at least one gas flow applied to increase the temperature within thesubstantially decompressive spray may also comprise providing heated gasor air adjacent to the spray so as to be entrained into the forming andformed spray. One method is to distribute the heated gas flow through atubular distribution system that discharges the heated gas flowsymmetrically to the spray in the vicinity of the spray tip. Forexample, the distribution system may consist of four discharge tubespositioned with two outlets on each side of the spray fan in thevicinity of the spray orifice. The at least one gas flow may alsocomprise blowing heated gas or air from a heater to the spray. Othermethods may also be used.

As used herein, "solid particulates" are particulates that aresubstantially physically rigid particles or powders which may containresidual solvent and which includes highly viscous liquid particles thatlack crystalline structure but which do not coalesce significantly withone another when in contact and do not flow perceptibly. The solidparticulates are not required to have a particular shape or form andthey may be porous.

In general, the average size of the solid particulates produced from thesubstantially decompressive spray can be controlled by adjusting thecompressed fluid concentration, spray temperature and pressure, andsolvent level in the solvent-borne composition. Solid particulates areproduced which have an average size of about one micron or greater. Theaverage size is preferably greater than about 5 microns, more preferablygreater than about 10 microns, still more preferably greater than about15 microns, and most preferably greater than about 20 microns. Ingeneral, larger particle sizes require more time to evaporate thesolvent, the actual time depending upon the morphology of theparticulates, the relative evaporation rates of the solvents, and thespray temperature and mixing characteristics. The average particle sizein general should be less than about 200 microns, preferably less thanabout 150 microns, more preferably less than about 100 microns, stillmore preferably less than about 75 microns, and most preferably lessthan about 50 microns. The optimal particle size will depend upon theparticular application requirements for the solid particulates. Theoptimal particle size for highly porous particulates will generally behigher than that for particulates with low porosity.

The decompressive spray produces uniform atomization that can producesolid particulates and powders that have a relatively narrow particlesize distribution, which is often desirable to improve their performancein applications. Not only can the particle size distribution be narrowat a point in the spray, but the average particle size can be veryuniform across the spray pattern, which gives a narrow overall particlesize distribution for the entire spray, become some regions are notover-atomized or under-atomized. Nonuniform atomization across the spraypattern is frequently a problem with air and airless spray methods.

The width or narrowness of a particle size distribution can be given byits span. The span is defined as (D₀.9 -D₀.1)/D₀.5, where D₀.5 is thesize for which 50% of the particle volume has smaller (or larger) sizeand equals the average particle size, D₀.1 is the size for which 10% ofthe particle volume has smaller size, and D₀.9 is the size for which 10%of the particle volume has larger size. Preferably, the particle sizedistribution has a span less than about 2.0, more preferably less thanabout 1.8, still more preferably less than about 1.6, and mostpreferably less than about 1.4. A narrower span has a smaller percentageof particles that may be too small or too large for a given application.The desirable span will vary with the application.

A narrow particle size distribution measured for solid particulatesproduced by the methods of the present invention is shown in FIG. 1. Thesolvent-borne composition contained 30% non-volatile materials fractioncomprising solid acrylic polymer and 70% solvent fraction comprisingmethyl ethyl ketone. The liquid mixture contained 42.5% compressedcarbon dioxide fluid and was sprayed at 60° C. and 1600 psi, at whichtemperature and pressure carbon dioxide is a supercritical fluid. Theliquid mixture was heated to 60° C. to offset the cooling effect of thedecompressing carbon dioxide in order to increase the evaporation rate.The decompressive spray in ambient air produced a spray dried powderhaving a very narrow particle size distribution with a low span of 1.3and an average particle size of 21 microns. Only 11% of the particles byvolume were below 10 microns and only 10% were above 36 microns.

The distance from the spray tip at which solid particulates are producedthroughout the decompressive spray pattern will depend upon manyvariables such as the orifice size, particle size, relative evaporationrates of the solvents, the solvent level in the solvent-bornecomposition, spray temperature and pressure, the turbulent mixingintensity in the spray, application of heated gas, use of a pre-orifice,and other factors. The distance will depend upon the actualcharacteristics of a particular application. Increasing the level ofturbulent mixing or the temperature in the spray or both generallyproduces solid particulates more uniformly throughout the spray patternat a shorter distance. In general, the distance increases as the averagerelative evaporation rate of the solvent fraction decreases, until atunsuitably low average relative evaporation rates the spray remainsliquid for excessively long distances. Although some solvent-bornecompositions produce solid particulates uniformly throughout the spraypattern at short distances of about 6 inches to about 24 inches, othercompositions require longer distances. A distance greater than about 24inches is generally preferred for collecting the solid particulates fromthe spray, although shorter distances may be used for fast dryingsprays. More preferably the distance is greater than about 36 inches andmost preferably greater than about 48 inches, to provide moreevaporation time. The solid particulates and powders may be collectedfrom the spray by any means suitable for separating fine particulatesfrom a flow of gas or air, such as by a cyclone separator, filtration,electrostatic deposition, and other means known to those skilled in theart.

If desired, the solid particulates and powder may be treated to removeresidual solvent, such as by fluidization or mixing with air to stripsolvent, drying, or by passing air through a storage container or bin.

This invention may also be used to form coating powder by spray drying aliquid precursor coating-powder composition containing 1) a solidsfraction containing the dry ingredients of a coating powder and which iscapable of forming powder by solvent evaporation when sprayed as adecompressive spray; and 2) a solvent fraction which is at leastpartially miscible with the solids fraction and which is sufficientlyvolatile to render said precursor coating-powder composition capable offorming powder when sprayed as a decompressive spray. For spraying, theprecursor coating-powder composition is admixed with at least onecompressed fluid to form a liquid mixture in a closed system, thecompressed fluid being in an amount which renders the liquid mixturecapable of forming a substantially decompressive spray. The liquidmixture is then sprayed at a temperature and pressure that gives asubstantially decompressive spray by passing the mixture through anorifice into an environment suitable for forming coating powder bysolvent evaporation. The aforementioned teachings pertaining to formingsolid particulates are understood to pertain, where applicable, toforming coating powders by the methods of the present invention, as willbe understood by those skilled in the art, with the following discussionbeing particular to coating powders.

The solids fraction of the precursor coating-powder composition containsthe dry ingredients of a coating powder. As used herein, it isunderstood that the term "coating powder" includes coating powders andpowder-coating compositions used for powder coating of substrates, aswell as powder components for liquid coating compositions such asadditives. As known by those skilled in the art, the dry ingredients ofcoating powders for powder-coating applications generally may compriseat least one thermosetting or thermoplastic polymer, a curing orcross-linking agent for thermosetting systems, plasticizers,stabilizers, flow additives, pigments, and extenders.

For use as a powder coating, the polymer should have low melt viscosity,to provide a smooth continuous film; good adhesion to the substrate;good physical properties such as toughness and impact resistance; lightcolor; heat, chemical, and weathering resistance; and storage stability.Thermosetting coating powders generally use polymers that are cured byaddition reactions instead of condensation reactions. The glasstransition temperature of coating-powder thermosetting polymers shouldbe high enough to prevent individual particles from sintering or fusingduring transportation and storage. Refrigerated storage allows polymerswith lower glass transition temperatures to be used. For roomtemperature storage, preferably the glass transition temperature isabove about 40° C., more preferably above about 50° C. Polymers suitablefor use in the coating powders of the present invention in generalcomprise those used in conventional powder coating and in particularinclude epoxies, polyesters, acrylics, polyurethanes, epoxy-polyesterhybrids, cellulosics, vinyls, polyamides, and hybrid polymers thereof.Other polymers such as polyolefins may also be used. Polymer types thatare not presently used for powder coating, because they are notcompatible with cryogenic grinding or mechanical milling, may also beused with this invention. Thermosetting systems may use any of thecuring or cross-linking agents commonly used in powder coating, such asblocked isocyanate polymers and triglycidyl isocyanurate.

Here again, the solids fraction must be a sufficiently high fraction ofthe precursor coating-powder composition to be capable of forming acoating powder by solvent evaporation when sprayed by the decompressivespray, and to form powder particles of sufficient size, but it must notbe so excessively high as to be unable to form a substantiallydecompressive spray or to cause overly large particles to form. Ingeneral, the solids fraction should generally be greater than about 10%by weight of the precursor coating-powder composition, preferablygreater than about 15%, more preferably greater than about 20%, and mostpreferably greater than about 25%. However, the solids level used willdepend upon the properties of the polymer system and other componentsused and the appropriate level must generally be determinedexperimentally. Due to lower polymer molecular weights, thermosettingsystems can generally be sprayed at higher solids levels thanthermoplastic systems. For such systems, solids levels may be greaterthan about 40%, preferably greater than about 50%, although lower levelsmay be used as well. For applications wherein the coating powderproduced by the decompressive spray is applied directly to a substrate,polymers with lower molecular weight and lower glass transitiontemperature may be used than are used in conventional coating powderswhich must be stored and transported. Therefore, the solids levelachievable can be correspondingly higher, and may be 70% to 90% orhigher, depending upon the application requirements and the viscosity.Although higher and lower viscosities may be used, the precursorcoating-powder composition will generally have a viscosity of from about500 to about 5000 centipoise, preferably from about 800 to about 3000centipoise, as measured at a temperature of about 25° C.

The solvent fraction is chosen to be at least partially miscible withthe solids fraction and to be sufficiently volatile. Any of theaforementioned solvents may be used depending upon their suitability andsolubility characteristics for the particular system. Generally at leastone solvent is an active solvent for the polymer used. Here again, thesolvent fraction desirably has an average relative evaporation rategreater than about 70, preferably greater than about 85, more preferablygreater than about 105, still more preferably greater than about 140,and most preferably greater than about 175. The average relativeevaporation rate is preferably less than about 4000, more preferablyless than about 3000, and most preferably less than about 2000.

The at least one compressed fluid is preferably carbon dioxide or ethaneand it is preferably a supercritical fluid at the temperature andpressure at which the liquid mixture is sprayed. At least one gas flowmay be applied to the substantially decompressive spray to increase therate of turbulent mixing or the temperature within the spray or both, asdiscussed previously.

For use in powder coating, the coating powders formed by the methods ofthe present invention desirably have an average particle size greaterthan about 10 microns, preferably greater than about 15 microns, andmost preferably greater than about 20 microns. The average particle sizeis preferably less than about 125 microns, more preferably less thanabout 100 microns, still more preferably less than about 75 microns, andmost preferably less than about 50 microns. The coating powdersdesirably have a narrow particle size distribution, preferably with aspan less than about 2.0, more preferably less than about 1.8, stillmore preferably less than about 1.6, and most preferably less than about1.4.

The coating powder thus formed may then be deposited on to a substrate,either directly or indirectly, and the substrate heated to form acoating film thereon, as known by those skilled in the art.

In the field of heterogeneous catalysis, there is a continuing need formorphologically improved catalyst supports and catalyst precursors. Theperformance of a catalyst is frequently affected by its morphologicalform. One step in the formation of a component of a catalytic materialoften consists of drying the material, and imparting this desired shapeupon the material as part of the drying process. Morphological featuressuch as shape, particle size, particle size distribution, porosity, andcrystallinity can be controlled, to a greater or lesser form, viatechniques such as crystallization, impregnation, or spray drying.

While the following discussion will focus on olefin polymerizationcatalyst systems, those skilled in the art of catalysis will appreciatethat the methods of this invention may be applied to other catalystsystems as well, and the methods are not limited to olefinpolymerization.

Spray drying is particularly advantageous for olefin polymerizationcatalyst systems, in that spherical or at least reasonably roundparticles of reasonably uniform size can frequently be obtained on alarge scale. A solution or slurry of either an inert support, a reactivesupport, or a catalyst precursor can be spray dried. Inert carriers areexemplified by microspheroidal silicas, reactive carriers by magnesiumsalts such as magnesium halides or magnesium hydrocarbyl carbonates, andcatalyst precursors by magnesium halide/titanium halide/electron donoradducts.

The thermal spray-drying processes are typically conducted in solventssuch as water, or organic solvents such as alcohols, ethers, or esters.The material to be spray-dried is at least partially dissolved, often inthe presence of inorganic or organic fillers. A hot solution or slurrycontaining the solid component to be spray-dried is typically ejectedfrom an orifice in the form of a spray, and the liquid particle is madeto dry during flight via evaporation. The droplet must be substantiallyrigid within a few seconds so as not to be deformed or destroyed uponimpact.

Spray-drying of these materials is, however, not without problems. Theenergy for rapid evaporation of the solvent component is typicallysupplied in the form of heat, and the particles are solidified viaevaporation. The solutions thus have to be as hot as possible, which inthe case of organic solvents and organometallic reagents may lead todecomposition, undesirable side reactions, or to premature precipitationof reaction products. Even when higher temperatures are feasible,process limitations interfere with successful spray drying. If the heatcapacity of the organic solvent is low, not enough thermal energy can beimparted to allow full drying; if the vapor pressure of the solvent istoo low, not enough evaporation will take place in the few secondsbefore the liquid droplets hit the walls of the spray drier. Duringspray drying at a large commercial scale, removal of the evaporatedsolvent is becoming a greater and greater challenge for process andenvironmental reasons. Organic solvents can frequently not be captured,removed, and confined rapidly enough, so that the throughput suffers. Inthe case of spray drying with flammable liquids, large amounts of inertgases such as nitrogen are required for the solvent removal process.

Spray drying imposes limitations on the physical form of the particle aswell. One problem arises from the fact that the solvent comprises mostof the volume of the droplet, that is, the solids content of thesolution is low. The size of the liquid droplets cannot be increasedbeyond a certain size without having the droplet fall apart. Loss of themajor volume fraction of that droplet on evaporation frequently leads toresidual solid particles having a size much smaller than desired. Othermeans of increasing the amount of solids in a droplet also havelimitations. Increasing the solids content of a solution are frequentlynot feasible, because the material is insufficiently soluble or theviscosity becomes too high to spray the material. Increases in thesolids content of a slurry leads to more frequent clogging of piping andinstrumentation, as well as to poorly dispersed product. While thematerial science aspects of the drying process are not sufficientlyunderstood in all cases, it can nevertheless be observed that thethermal evaporation process leads to non-uniform drying of the particle,as seen by skin formation (hollow shells), cracked particles, anddifferential precipitation of chemically different components within theparticle.

In contrast, by using compressed fluids according to the methods of thepresent invention, higher solids level can be achieved because lesssolvent is required for spraying; the evaporation rate can be increased;less thermal energy is required; lower spray temperature keeps thematerial stable; less high-boiling solvent need be removed; increasedsolubility may be obtained, especially in the presence of polymericbinders; less solvent need be recovered from the effluent; and lesspurge gas may be required. Furthermore, unique particle morphology maybe formed, depending upon the composition used, and the catalystsupports may have a narrower particle size distribution.

Two types of reactive magnesium-containing supports are spray dried.Formation of magnesium hydrocarbyl carbonates from magnesium hydrocarbyloxides and gaseous carbon dioxide is well known in the art (see U.S.Pat. No. 4,923,446). The magnesium hydrocarbyl carbonates can berepresented by the formula Mg(OR)(OR').xCO₂, wherein each of R and R'represent alkyl or aryl groups, and x has a value of from 0.1 to 2.0.This material is believed to be made up of a mixture of two, andpossibly more, components (H. L. Finkbeiner and G. W. Wagner, J. Org.Chem. 28: 215, 1963). These components include a monoalkoxymonocarbonate and a dicarbonate of formula Mg(OCOOR)₂. Among othercatalytic applications, these supports find use as components ofgas-phase olefin polymerization catalysts, as described in U.S. Pat.Nos. 4,540,679 and 4,771,024. Thermal spray-drying of magnesium alkylcarbonate supports has been disclosed in U.S. Pat. No. 4,771,024. Thematerials frequently are unstable towards decomposition to mixedalkoxides and carbonates at above room temperature. Solutions ofmaterials with R=methyl are only fully stable at elevated pressure;ethanol solutions with the R=ethyl material begin to decompose at theemployed conventional spray-drying temperatures of 70° C. to 100° C.when nitrogen gas is used at inlet temperatures of 100° C. to 140° C.Ethanol solutions with Mg content above approximately 4% are too viscousfor conventional spray drying on a commercial scale without undergoingpartial decomposition, because the spray-drying temperature must be toohigh.

Magnesium chloride solvates of electron donors such as alcohols orethers are well known in the art of olefin polymerization, such asdescribed in U.S. Pat. Nos. 4,124,532 and 4,684,703, and theirspray-drying, either by themselves (U.S. Pat. Nos. 3,953,414 and4,111,835) or as adducts with titanium halides, are also known (U.S.Pat. No. 4,293,673). Solutions in tetrahydrofuran are especiallytroublesome, in that the solubility of magnesium chloride intetrahydrofuran decreases by a factor of two between room temperatureand 65° C. due to the undesired precipitation of the material in a lesssoluble and presumably polymeric form (K. Handlir, J. Holecek, and L.Benes, Collection of Czechoslovak Chem. Commun. 50: 2422, 1985). Amethod for decreasing the feasible spray-drying temperature andincreasing solubility thus is highly desirable.

This invention may be used to form catalysts, catalyst supports, orcatalyst precursors for heterogenous catalysis by spray drying a liquidprecursor catalyst composition containing 1) a solids fractioncontaining the dry ingredients of a catalyst, catalyst support, orcatalyst precursor and which is capable of forming particulates bysolvent evaporation when sprayed as a decompressive spray; and 2) asolvent fraction which is at least partially miscible with the solidsfraction and which is sufficiently volatile to render said precursorcatalyst composition capable of forming particulates when sprayed as adecompressive spray. For spraying, the precursor catalyst composition isadmixed with at least one compressed fluid to form a liquid mixture in aclosed system, the compressed fluid being in an amount which renders theliquid mixture capable of forming a substantially decompressive spray.The liquid mixture is then sprayed at a temperature and pressure thatgives a substantially decompressive spray by passing the mixture throughan orifice into an environment suitable for forming particulates bysolvent evaporation. The aforementioned teachings pertaining to formingsolid particulates are understood to pertain, where applicable, toforming catalysts, catalyst supports, and catalyst precursors by themethods of the present invention, as will be understood by those skilledin the art, with the following discussion being particular to thesecatalyst materials.

The solids fraction of the precursor catalyst composition contains thedry ingredients of the catalyst, catalyst support, or catalyst precursorand may in general comprise at least one compound capable of functioningas a solid particulate catalyst, catalyst support, or catalystprecursor, which are known to those skilled in the art. Generally the atleast one compound will comprise an inorganic compound or anorganometallic compound. A polymeric compound such as a thermoplasticpolymer may also be used as a binder in the catalyst support. Theingredients of the catalyst, catalyst support, or catalyst precursor maycomprise any of the aforementioned materials used for olefin catalysis,including magnesium hydrocarbyl carbonates and magnesium chloride.

Here again, the solids fraction must be a sufficiently high fraction ofthe precursor catalyst composition to be capable of forming aparticulates by solvent evaporation when sprayed by the decompressivespray, and to form particulates of sufficient size, but it must not beso excessively high as to be unable to form a substantiallydecompressive spray or to cause overly large particulates to form. Thesolids fraction should generally be greater than about 15% by weight ofthe precursor catalyst composition, preferably greater than about 20%,more preferably greater than about 25%, and most preferably greater thanabout 30%. The appropriate amount will depend upon the physical andchemical characteristics of the particular solids fraction, such asmolecular weight and solubility. The solids fraction should generally beless than about 90% by weight of the precursor catalyst composition,preferably less than about 80%, more preferably less than about 70%, andmost preferably less than about 60%. Although higher and lowerviscosities may be used, the precursor catalyst composition willgenerally have a viscosity of from about 200 to about 5000 centipoise,preferably from about 500 to about 3000 centipoise, more preferably fromabout 800 to about 2000 centipoise, as measured at a temperature ofabout 25° C.

The solvent fraction is chosen to be at least partially miscible withthe solids fraction and to be sufficiently volatile. Higher solubilityis preferred. The solvents are preferably compatible with preserving thecatalyst activity and stability of the catalyst material. Any of theaforementioned solvents may be used, depending upon their solubility andsuitability for the particular catalyst system. If a polymeric compoundis included in the solids fraction, preferably at least one activesolvent for the polymer is used. As aforementioned, for catalystmaterials used for olefin catalysis, the preferred solvents are alcoholssuch as ethanol; ethers such as tetrahydrofuran (THF); and esters. Hereagain, the solvent fraction desirably has an average relativeevaporation rate greater than about 70, preferably greater than about85, more preferably greater than about 105, still more preferablygreater than about 140, and most preferably greater than about 175. Theaverage relative evaporation rate is preferably less than about 4000,more preferably less than about 3000, and most preferably less thanabout 2000.

The at least one compressed fluid is preferably compatible withpreserving the catalyst activity and stability of the catalyst material.The preferred compressed fluid may change with catalyst system. Ingeneral carbon dioxide or ethane are preferred, but ethylene, propane,or propylene, or a mixture, might be preferred for catalyst materialsprepared for olefin catalysis in order to obtain synergy orcompatibility with the polymerization operation.

The compressed fluid is preferably a supercritical fluid at thetemperature and pressure at which the liquid mixture is sprayed.Although higher spray temperature is favored for more rapid solventevaporation from the spray, the temperature must be compatible withmaintaining catalyst activity, because some catalyst materials asaforementioned are sensitive to heat, particularly when in solvent.Therefore, the lowest spray temperature that gives a desirabledecompressive spray and proper solvent evaporation is generallypreferred, which will depend upon the particular system used.

The catalysts, catalyst supports, and catalyst precursors formed by themethods of the present invention in general desirably have an averageparticle size greater than about 10 microns, preferably greater thanabout 15 microns, and more preferably greater than about 20 microns. Forsome catalyst systems, an average particle size above about 25 micronsis still more preferable, whereas for other systems, such as with highlyporous particles, larger particles with an average particle size aboveabout 40 microns are most preferable. In general, the average particlesize is preferably less than about 200 microns, more preferably lessthan 150 microns, and still more preferably less than about 125 microns.For some catalyst systems, the average particle size is desirably lessthan 100 microns, and for other systems less than about 70 microns. Themost favorable particle size will depend upon the particularapplication. The catalysts, catalyst supports, and catalyst precursorspreferably have a narrow particle size distribution, as aforementioned.

To maintain catalyst activity and stability, for some moisture-sensitivecatalyst systems a spray environment is preferred that has very lowhumidity or more preferably is moisture free. For oxygen-sensitivecatalyst systems, the spray environment preferably has a low-oxygencontent or more preferably is oxygen-free, such as a nitrogenatmosphere.

It has also been discovered that a liquid precursor catalyst compositionhaving a solids fraction that contains at least one organic polymer andat least one inorganic or organometallic compound, when sprayed withcompressed fluid, may form catalyst, catalyst support, or catalystprecursor particulates having novel and useful morphology and particlesize. The individual particulate comprises an aggregate of solidmicroparticulates containing the at least one inorganic ororganometallic compound, such as a microcolloidal precipitate, which areat least partially enclosed in a polymeric shell. The aggregation ofmicroparticulates provides for a porous interior and the partially open,thin outer polymeric shell allows penetration of reactants to theinterior and enables desirable larger particle sizes to be formed.

For example, a precursor catalyst composition containing 20% (by weight)of a magnesium ethyl carbonate composition containing fumed silica, 20%solid acrylic polymer, 30% ethanol, and 30% ethyl acetate was sprayedwith 37% carbon dioxide in the liquid mixture at 60° C. and 1800 psiginto ambient air. Electron microscope photographs showed that thecatalyst support particulate thus formed comprised a porous aggregate ofseveral solid microparticulates, which would contain the precipitatedmagnesium hydrocarbyl carbonate, all partially enclosed in an acrylicpolymeric shell. The particle size distribution obtained is shown asdistribution B in FIG. 2, where it is compared with conventionaldistribution A, which was obtained by thermal spray drying, using arotary atomizer, of the magnesium ethyl carbonate composition inethanol. The catalyst support particle sizes obtained with thecompressed fluid are desirably substantially larger than the sizes forthe conventional catalyst support. The decompressive spray also produceda narrower, monomodal particle size distribution than the broad, bimodaldistribution produced by conventional spray drying, which also containedan undesirably large fraction of particles less than 10 microns in size.Furthermore, the conventional spray drying required a low solids levelbelow 8% and a high temperature above 100° C. in a hot nitrogenatmosphere in a drying chamber.

This invention may also be used to form solid particulates or powder byspray drying a liquid water-borne composition containing 1) anonvolatile materials fraction which is solid or capable of becomingsolid, which is capable of being sprayed, and which is capable offorming solid particulates by evaporation when sprayed as adecompressive spray; and 2) a solvent fraction containing at leastwater; which is sufficiently volatile to render, and contains water inan amount which renders, said water-borne composition capable of formingsolid particulates when sprayed as a decompressive spray. For spraying,the water-borne composition is admixed with at least one compressedfluid to form a liquid mixture in a closed system, the compressed fluidbeing a supercritical fluid at the temperature and pressure at which theliquid mixture is sprayed and which is substantially present in theliquid mixture as a finely dispersed liquid compressed fluid phase, inan amount which renders said liquid mixture capable of forming asubstantially decompressive spray. The liquid mixture is sprayed at atemperature above about 40° C., preferably above about 50° C., and morepreferably above about 55° C., and at a pressure, preferably above about1200 psig, more preferably above about 1400 psig, that gives asubstantially decompressive spray by passing the mixture through anorifice into an environment suitable for forming solid particulates byevaporation, preferably having a low humidity level. Because water has ahigher heat of evaporation than organic solvents, a higher spraytemperature is generally required. The water fraction preferablycontains at least 35% water by weight. Here again, the nonvolatilematerials fraction must be a sufficiently high fraction of thewater-bone composition to be capable of forming solid particulates byevaporation when sprayed by the decompressive spray. The water-bonecomposition may be a solution or a dispersion. The solids fraction maycontain at least one polymer which is a water-dispersible polymer or awater-soluble polymer. The solids fraction may also be a detergentcomposition or it may contain at least one other water-soluble organicmaterial, or it may contain at least one water-soluble inorganiccompound. The compressed fluid is preferably rendered capable of forminga liquid phase at supercritical temperature and pressure by thewater-bone composition containing at least one component, such as asolvent, that is miscible with the compressed fluid. As disclosed incopending U.S. patent application Ser. No. 128,880, (now U.S. Pat. No.5,419,487) filed Sep. 29, 1993, water-bone compositions containing asufficient amount of a finely dispersed liquid compressed fluid phasecan form a decompressive spray in the absence of significant compressedfluid solubility in the water phase. Although higher viscosities may beused if a substantially decompressive spray is formed, the water-bonecomposition will generally have a viscosity below about 2000 centipoiseat a temperature of 25° C., preferably below about 1500 centipoise, morepreferably below about 1000 centipoise, and most preferably below about700 centipoise. Preferably, at least one gas flow is applied to thesubstantially decompressive spray to increase the rate of turbulenttaking or the temperature within the spray or both, in order to increasethe evaporation rate of the water. The aforementioned teachingspertaining to forming solid particulates from solvent-bone compositionsare understood to pertain, where applicable, to forming solidparticulates from water-bone compositions in the practice of thisinvention, unless indicated otherwise.

It has also been discovered that the decompressive spray can be used toapply drier coating films from water-bone coating compositions havingconventional water levels, due to enhanced evaporation of water in thespray. The water-borne coating composition sprayed 1) contains a waterlevel which renders the composition capable of being sprayedconventionally with no compressed fluid, 2) is capable of forming acoating on a substrate, and 3) contains a solvent fraction having atleast about 35 percent water by weight. For spraying, the water-bornecoating composition is admixed with at least one compressed fluid toform a liquid mixture in a closed system, the compressed fluid 1) beingsubstantially present in the liquid mixture as a finely dispersed liquidcompressed fluid phase and 2) being in an mount which renders the liquidmixture capable of forming a substantially decompressive spray. Theliquid mixture is sprayed at a temperature and pressure that gives asubstantially decompressive spray by passing the mixture through anorifice into an environment suitable for water evaporation and applyinga coating to a substrate. Preferably the gaseous environment has a lowhumidity level.

Surprisingly, even though the compressed fluid may have littlesolubility in the water-borne coating composition, by using a finelydispersed liquid compressed fluid phase in the liquid mixture, the sprayundergoes a transition from a liquid-film spray to a decompressivespray, as the compressed fluid level is increased or temperature isincreased, in a manner similar to that aforementioned for water-freecompositions having significant compressed fluid solubility. Preferredcompressed fluids are carbon dioxide and ethane.

As used herein, it will be understood that the term "water-borne coatingcomposition" includes not only coating compositions used to formprotective or decorative coatings but also includes adhesives, releaseagents, lubricants, agricultural materials, and the like, which arecapable of being sprayed to deposit a coating on a substrate. Thewater-borne coating compositions that may be used with the presentinvention will typically contain at least one polymer which is awater-dispersible polymer or a water soluble polymer and which iscapable of forming a coating on a substrate. Applicable polymers includethermoplastic polymers, thermosetting polymers, crosslinkable filmforming systems, two-component reactive polymer systems, and mixturesthereof, typically used in water-borne coating compositions sprayedconventionally with no compressed fluid. The polymers may be solidpolymers or liquid polymers, and they may be dissolved, dispersed, oremulsified in the solvent fraction.

In particular, the polymers include acrylics, polyesters, polyvinylresins such as polyvinyl acetate, alkyds, polyurethanes, epoxies,phenolic resins, cellulosic polymers such as methyl cellulose,hydroxyethyl cellulose, carboxymethyl cellulose, and nitrocellulose,amino polymers such as urea formaldehyde and melamine formaldehyde,polyethylene glycols and polypropylene glycols, polyomides, natural gumsand resins, polymers containing silicon, and the like.

In addition to the polymers, the composition may contain conventionaladditives which are typically utilized in water-borne coatings. Forexample, pigments, pigment extenders, metallic flakes, fillers,surfactants, wetting agents, emulsifying agents, dispersing agents,thickeners, anti-foaming agents, coalescing agents, driers, ultravioletabsorbers, biocides, pH buffers, neutralizers, cross-linking agents,plasticizers, and mixtures thereof, may all be utilized.

In addition to the water, the solvent fraction may contain one or moreorganic solvents. The organic solvent may perform a variety offunctions, such as to solubilize the polymer and other components, togive proper flow characteristics such as leveling, to adjust the dryingrate, aid pigment dispersion, and the like. Generally the preferredorganic solvents are water soluble, such as alcohols, glycol ethers,acetone, methyl ethyl ketone, and the like, that are typically used inwater-borne coating formulations. Coupling solvents such as ethyleneglycol ethers, propylene glycol ethers, and the like may also be used.The selection of a particular solvent fraction to form a givenwater-borne coating composition is well known to those skilled in theart of coatings.

Preferably, the water-borne coating composition has at least 70% of thewater content used to spray the composition without compressed fluid,more preferably at least 80%, still more preferably at least 90%, andmost preferably at least about 95%.

Preferably, the water-borne composition has a viscosity at a temperatureof about 25° C. of less than about 200 centipoise, more preferably lessthan about 150 centipoise, still more preferably less than about 100centipoise, and most preferably less than about 75 centipoise

Conventional spray methods by which the water-borne coating compositionwould be sprayable without compressed fluid include air spray,high-volume low-pressure spray (HVLP), airless spray, air-assistedairless spray, and rotary atomizers.

When carbon dioxide dissolves in water, some of the carbon dioxide formscarbonic add, which increases the acidity and lowers the pH of thesystem. Therefore, when carbon dioxide is used as the compressed fluidwith water-borne coating compositions that are sensitive to lowered pHlevel, particularly to acidic pH levels, preferably the pH of the liquidmixture is controlled to prevent polymer precipitation when the carbondioxide is admixed with the water-borne composition. Preferably the pHis controlled by using a pH buffer. Buffers are commonly used inwater-borne coating compositions to maintain the pH at a desirablelevel, as is known to those skilled in the art. One example of a bufferis a carbonate/bicarbonate buffer, which regulates the pH at about 10.The pH may also be controlled by using alkali or other basic materialssuch as ammonia, sodium hydroxide, calcium carbonate, and other salts.

The liquid compressed fluid phase is preferably finely dispersed intothe liquid mixture by vigorously agitating or mixing the liquid mixtureas the compressed fluid is admixed with the water-borne coatingcomposition. How the materials are admixed is not critical to thepractice of this invention, provided that the liquid compressed fluidphase becomes substantially finely dispersed in the liquid mixture.Static or powered mixers may be used. Forming and maintaining the finelydispersed liquid compressed fluid phase in the liquid mixture may beaided by using a dispersion, emulsifying, or stabilization agent for thecompressed fluid in the practice of the present invention. Such agentsare generally surfactant materials, such as TERGITOL® nonionicsurfactant NP-10, that are used to produce more or less stable mixturesof immiscible liquids such as hydrocarbons in water (Martens, Charles R,Water-Borne Coatings, Van Nostrand Reinhold, New York, 1981). Theypromote ease of mixing by reducing interfacial tension. The surfactantsgenerally comprise long chain molecules that have a hydrophilic end anda lipophilic end. Such surfactant materials are commonly used inwater-borne coating compositions that contain dispersed polymers, asknown by those skilled in the art. Such materials may also aid informing and maintaining a dispersion of the liquid compressed fluidphase, which typically has properties similar to a hydrocarbon material.As much as five percent or more based on the vehicle solid weight may berequired.

The amount of compressed fluid that is used in the liquid mixture shouldbe such that the liquid compressed fluid phase remains substantiallyfinely dispersed in the liquid mixture and gives proper atomization. Ifthe mount of compressed fluid is excessively high, larger than desirableagglomerations of the liquid compressed fluid can form in the liquidmixture, which can become more difficult to maintain as a uniformdispersion. Therefore, although larger quantities may be used, theamount of compressed fluid present in the liquid mixture is preferablyless than about 40 percent by weight, more preferably less than about 35percent, still more preferably less than about 30 percent, and mostpreferably less than about 25 percent. The amount of compressed fluidpresent in the liquid mixture should be at least an amount which rendersthe liquid mixture capable of forming a substantially decompressivespray. The amount required will depend upon the viscosity andrheological properties of the water-borne coating composition. Theliquid mixture preferably contains at least about 4 percent compressedfluid, more preferably at least about 6 percent compressed fluid, stillmore preferably at least about 10 percent compressed fluid, and mostpreferably at least about 15 percent compressed fluid.

Although higher pressures may be used, preferably the spray pressure isbelow about 3000 psi, more preferably below about 2000 psi. Preferablythe spray pressure is above about 50 percent of the critical pressure ofthe compressed fluid, more preferably above about 75 percent of thecritical pressure, and most preferably above the critical pressure.Preferably the pressure is high enough to enable the compressed fluid toform a liquid phase.

Preferably, the spray temperature of the liquid mixture is below about150° C., more preferably below about 100° C., and most preferably belowabout 80° C. Preferably, the spray temperature is above about 25° C.,more preferably above about 30° C., still more preferably above 40° C.,and most preferably above 50° C., to increase the evaporation rate ofthe water.

The liquid mixture is preferably sprayed at a temperature and pressureat which the compressed fluid is a supercritical fluid. The compressedfluid is preferably rendered capable of forming a liquid phase atsupercritical temperature and pressure by the water-bone compositioncontaining at least one component, such as a solvent, that is misciblewith the compressed fluid.

The water-bone coating composition preferably contains at least oneorganic solvent that is capable of being extracted from the water-bonecoating composition into the compressed fluid, thereby enabling saidcompressed fluid to form the liquid compressed fluid phase at thesupercritical temperature and pressure. It is understood that only aportion of the organic solvent, generally only a small portion, need beextracted from the water-borne coating composition in order to form theliquid compressed fluid phase.

Alternatively, the liquid mixture may contains in addition at least oneorganic solvent which is immiscible with the water-bone coatingcomposition; which is at least partially miscible with the compressedfluid under pressure; and which is present at least in an amount whichenables the compressed fluid to form the liquid compressed fluid phaseat the supercritical temperature and pressure. This is desirable whenthe water-bone coating composition does not contain an extractableorganic solvent or contains such solvent in insufficient quantity. It isparticularly useful for spraying water-bone coating compositionscontaining water-dispersible polymers. Such insoluble organic solventsare typically hydrocarbon solvents such as pentane, hexane, heptane,octane, decane, toluene, xylene, and the like, including branched andaromatic hydrocarbons, but other insoluble solvents may also be used.The insoluble solvents preferably have a relatively high relativeevaporation rate, preferably above about 100 (butyl acetate RER=100),such as pentane, hexane, and heptane, toluene, and the like, so that thesolvent readily evaporates during spraying. The insoluble organicsolvent should be used in a minimal amount that gives a sufficientamount of liquid compressed fluid phase for spraying, so as to minimizeorganic solvent emissions. Generally the amount of insoluble organicsolvent will be between about 2 percent and about 25 percent of thetotal weight of water-borne coating composition and insoluble organicsolvent, preferably between about 4 percent and about 20 percent, andmore preferably between about 5 and about 15 percent.

Here too, an elongated orifice passageway with the characteristicsaforementioned may be used to spray the liquid mixture.

Liquid spray droplets are produced which generally have an averagediameter of one micron or greater. Preferably, the droplets have averagediameters of about 5 to about 150 microns, more preferably from about 10to about 100 microns, still more preferably from about 15 to about 70microns, and most preferably from about 20 to about 50 microns.

The environment into which the water-borne coating composition issprayed is not narrowly critical. Preferably, the liquid mixture issprayed into air under conditions at or near atmospheric pressure. Othergaseous environments can also be used. The relative humidity shouldallow sufficient evaporation of water from the liquid spray in order toproduce desirable coating formation on a substrate. Thereforeexcessively high relative humidity should be avoided and low relativehumidity is preferred.

The liquid spray mixtures containing the compressed fluid may beprepared for spraying by any of the spray apparatus disclosed in theaforementioned patents or other apparatus. The spray apparatus may alsobe a UNICARB® System Supply Unit manufactured by Nordson Corporation toproportion, mix, heat, and pressurize coating compositions withcompressed fluids such as carbon dioxide for the spray application ofcoatings.

While preferred forms of this invention have been described, it shouldbe apparent to those skilled in the art that methods and apparatus maybe employed that are different from those shown without departing fromthe spirit and scope thereof.

EXAMPLE 1

Liquid polymeric compositions containing cellulose acetate butyratepolymer were prepared by dissolving solid Eastman Chemical CelluloseEster CAB-381-0.1 in different solvents for spraying with compressedcarbon dioxide fluid. The polymer had molecular weights of 45,260(M_(w)) and 19,630 (M_(n)).

The composition and carbon dioxide were mixed and sprayed on acontinuous basis by using the apparatus disclosed in FIG. 2 of U.S. Pat.No. 5,105,843. Carbon dioxide from a cylinder was pumped and regulatedto the spray pressure, and a mass flow meter measured the mass flow rateof carbon dioxide fed through a check valve to the mix point. Thecomposition was pumped from a tank and then metered by a precision gearpump. A gear meter measured the amount delivered through a check valveto the mix point. The speed of the gear pump was controlled by a signalfrom the mass flow meter to automatically produce the desired proportionof composition and carbon dioxide. The metering rate was adjusted by afeedback signal from the gear meter to correct pump inefficiency. Theliquid mixture of composition and carbon dioxide was mixed in a staticmixer and admixed with recycled liquid mixture. The circulation loop hada static mixer, piston-type accumulator, two heaters, filter, sightglass, spray gun, and circulation pump. A Nordson SCF-1 automatic spraygun was used with spray tip #123007, which had a 9-mil orifice size anda 10-inch fan width rating.

The first composition contained 30% polymer dissolved in 70% methylethyl ketone (by weight). The average relative evaporation rate (RER)was 631. A spray mixture having 45-46% carbon dioxide, a spraytemperature of 50° C., and a spray pressure of 1800 psig (gauge)produced a clear spray solution and a parabolic decompressive spray witha width of 12-14 inches. Spraying a test panel showed that dry powderwas produced throughout the spray pattern in the ambient air of thespray hood (about 25° C.) by a distances of 12 inches from the spraytip. A spray temperature of 60° C. produced dry powder by 9 inches.Spray dried powders were collected.

The second composition contained 40% polymer and 60% methyl ethylketone. A spray mixture with about 45% carbon dioxide at 50° C. and 1600psig produced a two-phase mixture, a decompressive spray, and dry powderby 12-inches distance. Spraying at 60° C. also produced dry powder by 12inches. Spray dried powder was collected and measured by a Malvern™dry-particle sizer. The powder had an average particle size of 24microns and a narrow distribution with only 12% of the particles (byvolume) below 10 microns and only 10% above 55 microns in size. Usingthe pre-orifice (#138344) for the spray tip produced dry powder by 9inches, which was collected.

The third composition contained 30% polymer and 70% butyl acetate. Theaverage RER was 100. A spray mixture with 49% carbon dioxide at 60° C.and 1600 psig produced a clear solution and a decompressive spray. Drypowder was produced throughout the spray pattern by 18 inches. Sprayingat 71° C. (solubility limit) produced dry powder by 12 inches. Spraydried powder was collected.

The fourth composition contained 30% polymer, 35% methyl ethyl ketone,and 35% methyl amyl ketone. The average RER was 75. A spray mixture withabout 30% carbon dioxide at 50° C. and 1600 psig produced an angularliquid-film spray that remained liquid and produced no dry powder. Using50% carbon dioxide produced a clear solution, a decompressive spray, anddry powder by 18 inches at both 50° C. and 60° C. For comparison, thecomposition was sprayed the same way at 60° C. with no carbon dioxide.This gave an angular liquid-film spray that remained liquid and producedno dry powder. For another comparison, using an air spray gun was tried,but the viscosity (780 centipoise) was too high to spray.

The fifth composition contained 30% polymer and 70% methyl amyl ketone.The average RER was 40. A spray mixture with about 55% carbon dioxidewas sprayed at temperatures up to 70° C. and pressures up to 1600 psig,all of which gave decompressive sprays that remained liquid and producedno dry powder.

The sixth composition contained 25% polymer, 23% methyl ethyl ketone,35% methyl amyl ketone, and 17% butyl CELLOSOLVE® acetate. The averageRER was 12. A spray mixture with about 36% carbon dioxide at 60° C. and1600 psig gave a decompressive spray that remained liquid and producedno dry powder.

EXAMPLE 2

Liquid polymeric compositions containing acrylic polymer were preparedby dissolving solid Robin & Haas Acryloid™ B-66 in different solvents.The polymer had molecular weights of 45,290 (M_(w)) and 24,750 (M_(n)).

The first composition contained 40% polymer and 60% acetone. The averageRER was 1440. A Nordson SCF-1 spray gun was used with tip #123007 andthe pre-orifice. A spray mixture with 35% carbon dioxide at 60° C. and1800 psig produced a fishtail liquid-film spray that remained liquid.Using 43% carbon dioxide produced a clear solution (solubility limit), adecompressive spray, and dry powder in ambient air by 12-inchesdistance, which was collected.

The second composition contained 30% polymer and 70% acetone. A spraymixture with 40% carbon dioxide at 60° C. and 1800 psig produced atransition spray. Using 45% carbon dioxide produced a clear solution anda decompressive spray, and spray dried powder was collected. Forcomparison, the composition was sprayed the same way with no carbondioxide. This airless spray gave a liquid-film spray that produced wetcobweb fibers and no dry powder. For another comparison, a compositionof the polymer and acetone having a typical air spray viscosity of 93centipoise was sprayed with an sir spray gun. This air spray alsoproduced cobweb fibers and no powder.

The third composition contained 30% polymer and 70% methyl ethyl ketone.The average RER was 631. First, the Nordson SCF-1 spray gun was usedwith tip #123007 and no pre-orifice. A spray mixture with 42.5% carbondioxide at 60° C. and 1600 psig produced a clear solution, adecompressive spray, and dry powder by 12-inches. Spray dried powder wascollected and the particle size was measured. The powder had an averageparticle size of 21 microns and a narrow distribution with only 11% ofthe particles (by volume) below 10 microns and only 10% above 36 micronsin size. For comparison, the composition was sprayed the same way withno carbon dioxide. This gave an angular liquid-film spray that producedwet cobweb fibers and no dry powder. For another comparison, acomposition of the polymer and methyl ethyl ketone having a typical airspray viscosity of 94 centipoise was sprayed with an air spray gun. Thisair spray also produced large fibrous cobwebs and no powder. Second, aGraco AA3000 air-assisted airless spray gun was used with spray tip#163-309, which has a 9-mil orifice. A spray mixture with 42.5% carbondioxide at 60° C. and 1600 psig produced a clear solution, adecompressive spray, and dry powder at a distance greater than 12 incheswhen no assist air was used. Using 20-psig atomization assist air, butno shaping air, produced dry powder (throughout the spray pattern) atless than 12 inches. Therefore increased turbulent mixing of surroundingair into the spray interior increased the evaporation rate. Heating theassist air to 30° C. and 40° C. gave dry powder at greater than 12inches, and to 50° C. and 60° C. gave dry powder at less than 12 inches.Heating the assist air at constant air pressure decreased the air massflow rate because the density was lower, but higher temperatureincreased volatility. Using 40 psig air at 23° C., 30° C., and 40° C.produced dry powder at less than 12 inches, and at 50° and 60° C.produced dry powder at less than 10 and 8 inches, respectively. Spraydried powder was collected at each condition.

The fourth composition contained 30% polymer and 70% methyl propylketone. The average RER was 269. The Nordson SCF-1 spray gun was usedwith tip #123007. A spray mixture with 42% carbon dioxide at 60° C. and1800 psig produced a clear solution and a decompressive spray. Using thepre-orifice produced dry powder at less than 18-inches distance. Usingno pre-orifice produced dry powder at greater than 18 inches. Spraydried powder was collected for each.

The fifth composition contained 30% polymer and 70% butyl acetate. Theaverage RER was 100. The Graco spray gun was used with tip #163-309. Aspray mixture with 43.5% carbon dioxide at 60° C. and 1600 psig produceda clear solution, a decompressive spray, and dry powder at greater than24-inches distance with no assist air and with atomization air at 20 and40 psig and 23°, 30°, 40°, and 50° C. Spray dried powder was collectedfor each case.

The sixth composition contained 40% polymer and 60% butyl acetate. Aspray mixture with 44% carbon dioxide at 60° C. and 1600 psig produced atwo-phase spray mixture and a decompressive spray. First, the Gracospray gun and tip #163-309 were used with no assist air and withatomization air at 20 and 40 psig and 23°, 30°, 40°, and 50° C. Second,the Nordson spray gun with spray tip #123007 and the pre-orifice wereused without and with entrainment air applied to the spray by four1/4-inch copper tubes positioned with two outlets on each side of thespray fan at a distance of one inch from the spray and one inch aboveand below the spray centerline. Air was used at 20 and 40 psig and 22°,30°, 40°, 50°, and 60° C. Dry powder was produced at greater than24-inches distance and spray dried powder was collected for each case.For comparison, the composition was sprayed with no carbon dioxide byusing the Nordson gun both with and without the pre-orifice at the sametemperature and pressure. This gave an angular liquid-film spray withheavy side jets that produced wet stringy material and a heavy centerjet that remained liquid and produced no dry powder.

The seventh composition contained 35% polymer and 65% methyl amylketone. The average RER was 40. A spray mixture with 42% carbon dioxideat 60° C. and 70° C. and 1600 psig gave a clear solution and adecompressive spray that remained liquid and produced no dry powder. Thesame results were obtained with 44% acrylic polymer and 56% methyl amylketone.

EXAMPLE 3

Liquid precursor coating-powder compositions containing an acrylicpowder coating polymer were prepared by dissolving solid S. C. JohnsonPolymer SCX-817 in different solvents. The polymer glass transitiontemperature was 68° C. and softening point was 120° C.

The composition and carbon dioxide were mixed on a batch basis by usingthe apparatus of Example 1 with no filter and one heater. Thecirculation loop was filled with spray mixture, the feeds closed, andpressure maintained by regulating nitrogen to the accumulator. A NordsonSCF-1 spray gun was used with spray tip #123006, which has a 9-milorifice and an 8-inch fan width rating.

The first composition contained 50% SCX-817, 25% methyl ethyl ketone,and 25% butyl acetate. The average RER was 173. First no pre-orifice wasused. A spray mixture with 20% carbon dioxide at 60° C. and 1800 psiggave a liqnid-film spray. Using 25% carbon dioxide gave a clear solution(solubility limit) and an angular spray with no visible liquid film.Increasing the temperature to 70° C. and pressure to 2050 psig gave atransitional spray. Using the pre-orifice gave a decompressive spraythat produced dry powder (throughout the spray pattern) in ambient airat less than 18-inches distance from the spray tip. Spray dried powderwas collected.

The second composition contained 45.0% SCX-817, 21.5% methyl ethylketone, 21.5% butyl acetate, and 12.0% acetone. The average RER was 214.Using the pre-orifice, a spray mixture with 23.7% carbon dioxide at 60°C. and 1800 psig gave a single-phase solution and a liquid-film spraythat remained liquid and produced no spray dried powder. Increasing thetemperature to 70° C. and the pressure to 2050 psig gave a transitionalspray. Using 26.5% carbon dioxide gave a well-dispersed two-phasemixture and a decompressive spray that produced dry powder by 12-inchesdistance. Heated air was then applied locally by directing a hot air gunat the spray at a distance of 8-10 inches from the spray tip. Spraydried powder was collected for both cases.

The third composition contained 45.0% SCX-817, 31.4% acetone, 15.3%methyl ethyl ketone, and 8.3% butyl acetate. The average RER was 426.Using the pre-orifice, a spray mixture with 31% carbon dioxide at 60° C.and 1600 psig gave a well-dispersed two-phase mixture and a narrowparabolic decompressive spray. Dry powder was formed at greater than24-inches distance, and spray dried powder was collected. Thenentrainment air at 40 psig and 21° C. was applied to the spray by thetubular distributor of Example 2. This changed neither the spray shapenor width, but increased turbulent mixing of air into the sprayinterior. Dry powder was now produced (throughout the spray pattern) atless than 18-inches distance, so greater turbulent mixing increased theevaporation rate. Next the entrainment air was heated to 60° C. Drypowder was still produced at less than 18-inches distance. The mass flowrate of the heated air was lower due to lower density, so mixing wasless intense, and the evaporation rate did not change appreciably,despite higher air temperature.

EXAMPLE 4

A liquid polymeric composition containing poly(vinyl chloride-vinylacetate) copolymer was prepared by dissolving 25% solid Union CarbidePolymer VYHH in 75% acetone. The composition was put into a 10-literstirred high-pressure heated autoclave and carbon dioxide to a 20% levelwas added from a weighed cylinder by a Haskel pump, which regulated thespray pressure, The spray mixture flowed from the bottom of theautoclave through a heated high-pressure hose to a Graco manual airlessspray gun with Binks spray tip #9-0970. A thermocouple at the spray gunmeasured the spray temperature as 50° C. A pressure of 500 psig gave aliquid-film spray having a visible liquid film and side jets. The sprayremained liquid and produced no spray dried polymer. Increasing thepressure to 700 psig increased the concentration of dissolved carbondioxide and gave a transitional nearly decompressive spray having noliquid film and no side jets, which produced spray dried polymer inambient air that was collected at a distance of about 24 inches from thespray tip.

EXAMPLE 5

This example first describes the preparation of magnesium ethylcarbonate used to prepare catalyst supports for olefin catalysis. Thencompositions used for conventional thermal spray drying and for sprayingwith carbon dioxide are compared.

The following experimental procedure (step 1) was used for thecarbonation of magnesium ethoxide to prepare a stock solution. Into a1900-liter glass-lined reactor (equipped with a turbine agitator) wereadded 150 kilograms (kg) of magnesium ethoxide and 532 kg of ethanolunder a nitrogen atmosphere (<10 ppmv water). The contents of thereactor were continuously stirred at about 50 rpm while carbon dioxidewas continuously bubbled through the mixture at a rate of about 20-25kg/hr, until 116 kg of carbon dioxide were fed. The reactor jackettemperature was maintained at below 35° C. for the duration of thecarbonation reaction. The exotherm resulting from the addition of carbondioxide caused the temperature of the mixture to rise about 5-10 degreesover a period of about 60 minutes. Additional carbon dioxide was addedto achieve the desired stoichiometry. At the end, the magnesium ethoxidehad completely dissolved in the ethanol to form a dear, viscous solutionunder a carbon dioxide atmosphere. Excess carbon dioxide was vented offand the mixture analyzed at 4.03% magnesium by weight. This mixture wasused as a stock solution for further dilution and addition of inertfiber.

The following is a comparative example not in accordance with thisinvention which describes conventional thermal spray drying of magnesiumethyl carbonate to form a catalyst support. Sufficient fumed silicahaving a particle size in the range of from 0.1 to 1 micron (CAB-O-SIL®TS-610, manufactured by Cabot Corporation) was added to the stocksolution prepared above (step 1). The mixture was stirred by a turbineagitator during this time and for several hours thereafter to thoroughlydisperse the silica in the solution. The temperature of the mixture washeld at 30° C. throughout this period and a nitrogen atmosphere (<5 ppmwater) was maintained at all times. Additional ethanol was added asneeded to achieve the desired magnesium content of the feed. Theresulting slurry was spray dried by using an 8-foot diameterclosed-cycle spray dryer equipped with a rotary atomizer. The atomizerspeed could be adjusted to produce particles with a wide range of sizes.The scrubber section of the spray dryer was maintained at approximately-4° C. Nitrogen gas was introduced into the spray dryer at inlettemperatures of 100°-140° C. and was circulated at a rate ofapproximately 1700 kg/hr. The magnesium ethyl carbonate/fumed silicaslurry was fed to the spray dryer at a temperature of about 35° C. andat a rate sufficient to yield an outlet gas temperature of approximately70°-100° C. The atomization pressure was slightly above atmospheric.Note that some parties decarbonation does occur at the temperature ofspray drying. Thermogravometric analyses have indicated that loss ofcarbon dioxide from a solid sample begins at temperatures as low asabout 80° C. The decarbonation, however is only partial under theseconditions.

The following describes compositions prepared for use with thisinvention in spraying magnesium ethyl carbonate compositions withcompressed carbon dioxide. In one case slurries of silica filler andmagnesium ethyl carbonate solution in ethanol were used to prepare thecompositions. In a second case, thermally spray-dried particles wereredissolved in ethanol. In this case there was noticeable uptake of gasupon exposure to carbon dioxide. In a third case, magnesium ethoxide wasslurried in ethanol, added to a unit for mixing with carbon dioxide inpreparation for spraying, and carboxylated in situ with carbon dioxide.In all cases, significantly higher magnesium content at significantlylower solution viscosity could be obtained at below 60° C. in mixedalcohol/supercritical carbon dioxide solutions than was possible in pureethanol.

EXAMPLE 6

Liquid precursor catalyst compositions containing a magnesium ethylcarbonate solution and fumed silica dispersion in ethanol were preparedby the procedures described in Example 5. The average RER was 330. Theapparatus and spray gun of Example 3 were used with spray tip #123007and no pre-orifice.

The first composition had a 40% solids level (by weight). A spraymixture with 10% carbon dioxide at 60° C. and 1800 psig gave an angularliquid-film spray that remained liquid and produced no spray driedmaterial. Spraying while continuously increasing the carbon dioxideconcentration caused the spray to transition to a parabolicdecompressive spray, which produced dry catalyst-support powder(throughout the spray pattern) in ambient air (circa 25° C.) at adistance of less than 14 inches from the spray tip.

The second composition had a 50% solids level. A spray mixture with 30%carbon dioxide at 30° C. and 1800 psig gave an angular liquid-film spraythat remained liquid and produced no spray dried material. Increasingthe temperature to 40° C. gave a transition spray. Increasing thetemperature to 50° C. gave a parabolic decompressive spray with a widthof about 14 inches, which produced dry powder in ambient air at lessthan 16-inches distance. The carbon dioxide was fully dissolved.Increasing the temperature to 60° C. gave a similar spray which produceddry powder at less than 12-inches distance. Spray dried catalyst-supportpowder was collected at both temperatures and stored under nitrogen.

The second composition was then sprayed by using compressed ethanefluid. A spray mixture with 10% ethane at 60° C. and 1800 psig gave anangular liquid-film spray that remained liquid and produced no spraydried material. Increasing the ethane concentration to 14.7% gave aparabolic decompressive spray that produced dry powder in ambient air atless than 12-inches distance. The ethane was fully dissolved. Spraydried catalyst-support powder was collected and stored under nitrogen. ANordson A7A airless spray gun with Cross-Cut™ tip #711354 was then usedat the same conditions to produce spray dried catalyst-support powder.

In contrast, the catalyst-support is spray dried conventionally by arotary atomizer in a heated spray chamber. This requires a very lowsolids level below about 8% in order to atomize the material. It alsorequires a heated spray chamber to fully evaporate the ethanol. However,it is desirable to much lower spray and drying temperatures because thematerial is heat sensitive.

EXAMPLE 7

The liquid precursor catalyst composition contained 20% solid magnesiumethyl carbonate/fumed silica (prepared as described in Example 5), 20%solid Acryloid™ B-66 acrylic polymer, 30% ethanol, and 30% ethylacetate. The average RER was 153. The apparatus and spray gun of Example3 were used with spray tip #123007 and no pre-orifice. At 60° C. and1800 psig, spray mixtures with 30%, 33%, and 37% carbon dioxide gave atransition spray, a nearly decompressive spray, and a decompressivespray, respectively. The carbon dioxide was fully dissolved. Spray driedcatalyst-support powder from the decompressive spray was collected andstored under nitrogen. The powder had an average particle size of 113microns and a narrow distribution compared to conventionally sprayedcatalyst, which is desirable for fluidized reactors. Electron microscopephotographs show that the catalyst particles have a unique particlestructure, consisting of small porous aggregates of solidmicroparticulates encapsulated in a thin fractured polymer shell.

EXAMPLE 8

This example describes magnesium chloride solids level that can be usedin liquid precursor catalyst compositions sprayed with carbon dioxide,in order to prepare catalyst supports for polyolefin catalysis, and howthe level compares with the lower level used in conventional thermalspray drying.

A clear 0.6M solution of magnesium chloride in tetrahydrofuran (THF) wasmixed with supercritical carbon dioxide at room temperature, and itsdissolution behavior was monitored via a glass port. The clear solutionhad turned cloudy at 60° C., but remained sufficiently colloidal andnon-viscous so that spraying could readily be conducted at thattemperature. In contrast, thermal spray drying of the 0.6Msolution/slurry in THF at 80°-110° C. produces unacceptable amounts ofchips and agglomerates, necessitating a decrease of the magnesiumchloride content to about 0.4M for thermal spray-drying withoutprecipitation. Spraying using supercritical carbon dioxide by themethods of the present invention would thus permit a 50% increase insolids level in this case.

EXAMPLE 9

The liquid water-bone polymeric composition contained 57.50% UnionCarbide CARBOWAX® PEG-8000, which is solid polyethylene glycol(molecular weight of 8000), 20.00% water, 11.25% acetone, and 11.25%methyl ethyl ketone. The average RER was 159. The apparatus and spraygun of Example 3 were used with tip #123007 and the pre-orifice. A spraymixture with 10% carbon dioxide at 40° C. and 1600 psig gave a finelydispersed liquid carbon dioxide phase that produced a narrow parabolicdecompressive spray. The spray deposited a wet film on a test panel at12 inches and a layer of tacky particles at 24-inches distance. At 50°C., the spray deposited a layer of tacky particles at 12-inchesdistance. Spraying at 60° C. produced dry powder at greater than 24inches in ambient air, and spray dried powder was collected. Nextentrainment air was applied to the spray from the tubular distributor ofExample 2. Heated air at 20 psig gave dry powder (throughout the spraypattern) at distances of more than 24 inches (20° C. and 30° C.), 18inches (40° C. and 50° C.), and 12 inches (60° C.). Heated air at 40psig gave dry powder at distances of more than 24 inches (21° C.), 24inches (30° C.), 18 inches (40° C.), 15 inches (50° C.), and 12 inches(60° C.). Spray dried powder was collected at each condition.

EXAMPLE 10

The liquid polymeric composition was a water-borne emulsion polymersystem containing a solid acrylic polymer. A mixture with a minor amountof hexane was used for spraying with ethane. Because hexane does notdissolve in water, the average RER was 83.

The apparatus and spray gun of Example 3 were used with spray tip#123007 and the pre-orifice. The tubular distributor of Example 2 wasused to supply heated air at 40 psig and 60° C. to the spray to increasethe evaporation rate. A spray mixture with about 20% ethane at 65° C.and 1600 psig gave a finely dispersed liquid ethane phase, whichproduced a narrow parabolic decompressive spray. Dry powder was produced(throughout the spray pattern) by a distance of 24 inches from the spraytip. Spray dried powder was collected.

EXAMPLE 11

A water-borne coating composition that gives a CARBOWAX® coating wasprepared from Union Carbide CARBOWAX® Polyethylene Glycol 8000, which isa water soluble solid polymer. The composition contained 35.5%polyethylene glycol, 56.6% water, and 7.9% methyl ethyl ketone (byweight). The composition had a typical air spray viscosity of 78centipoise (23° C.) and a conventional water content.

The apparatus and spray gun of Example 3 were used with spray tip#123007 and the pre-orifice. A spray mixture with 15% carbon dioxideproduced a decompressive spray at 40° C. and 1600 psig. At a spraytemperature of 50° C., the spray mixture contained a finely dispersedliquid carbon dioxide phase and produced a steady parabolic feathereddecompressive spray having an average droplet size of 38 microns, asmeasured by a Malvern™ 2600 droplet sizer. A wood test panel, 12-inch by12-inch in size, was sprayed with the decompressive spray, which applieda very uniform CARBOWAX® coating having no sags or runs, despite the lowviscosity, due to the high evaporation rate produced in the spray. Aspray temperature of 60° C. produced a feathered decompressive sprayhaving an average droplet size of 37 microns. A wood test panel wassprayed and a very uniform CARBOWAX® coating having no sags or runs wasapplied.

For comparison, the water-borne coating composition was sprayed by usingan air spray gun. This produced an air spray having an average dropletsize of 43 microns. However, despite having a feathered spray patternand having approximately the same average droplet size as thedecompressive sprays, the air spray applied poor coatings that were verynon-uniform to the wood test panels, because the coatings were muchwetter and less viscous due to the higher water content of the depositedcoating, which caused runs and sags even in relatively thinly appliedcoatings. Therefore the decompressive sprays deposited drier coatings,because more water evaporated in the decompressive sprays than in theair spray.

For another comparison, the water-borne coating composition was sprayedby an airless spray using the same spray tip, pre-orifice, and spraypressure, but with no carbon dioxide, at spray temperatures of 50° C.and 60° C. This produced liquid-film sprays with average droplet sizesof 37 and 34 microns (center of spray), respectively, that depositedvery wet coatings on wood test panels, which caused sags and runs evenin relatively thinly applied coatings in both cases. Therefore, despitehaving approximately the same average droplet sizes, the decompressivesprays deposited drier coatings, because more water evaporated than inthe conventional airless sprays.

We claim:
 1. A process for forming solid particulates whichcomprises:(1) forming a liquid mixture in a closed system, said liquidmixture comprising: (a) a solvent-borne composition comprising:(i) anonvolatile materials fraction which is solid or capable of becomingsolid, which is capable of being sprayed, and which is capable offorming solid particulates by solvent evaporation when sprayed; and (ii)a solvent fraction which is sufficiently volatile to render saidsolvent-borne composition capable of forming solid particulates whensprayed and which solvent fraction has an average relative evaporationrate greater than about 70; and (b) at least one compressed fluid in anamount which when added to (a) renders said liquid mixture capable offorming a substantially decompressive spray, wherein the compressedfluid is a gas at standard conditions of 0° Celsius and one atmospherepressure (STP); and (2) spraying said liquid mixture at a temperatureand pressure that gives a substantially decompressive spray by passingthe mixture through an orifice into an environment suitable for formingsolid particulates by solvent evaporation, wherein the spray has anaverage particle size greater than about one micron.
 2. The process ofclaim 1 wherein said at least one compressed fluid is selected from thegroup consisting of carbon dioxide, nitrous oxide, ethane, ethylene,propane, and propylene.
 3. The process of claim 1 wherein said at leastone compressed fluid is a supercritical fluid at the temperature andpressure at which said liquid mixture is sprayed and said liquid mixtureis heated to a temperature that substantially compensates for the dropin spray temperature that occurs due to expansion cooling of thedecompressing compressed fluid, in order to increase the evaporationrate of solvent from the spray.
 4. The process of claim 1 wherein thesolid particulates thus formed have a narrow particle size distribution.5. The process of claim 1 wherein at least one gas flow is applied tothe substantially decompressive spray to increase the rate of turbulentmixing or the temperature within the spray or both.
 6. A process forforming a coating-powder which comprises:(1) forming a liquid mixture ina closed system, said liquid mixture comprising: (a) a precursorcoating-powder composition comprising:(i) a solids fraction containingdry ingredients of a coating-powder and which is capable of formingpowder by solvent evaporation when sprayed; and (ii) a solvent fractionwhich is at least partially miscible with (i) and which is sufficientlyvolatile to render said precursor coating-powder composition capable offorming powder when sprayed; and (b) at least one compressed fluid in anamount which when added to (a) renders said liquid mixture capable offorming a substantially decompressive spray, wherein the compressedfluid is a gas at standard conditions of 0° Celsius and one atmospherepressure (STP); and (2) spraying said liquid mixture at a temperatureand pressure that gives a substantially decompressive spray by passingthe mixture through an orifice into an environment suitable for formingcoating powder by solvent evaporation.
 7. The process of claim 6 whereinsaid at least one compressed fluid is carbon dioxide or ethane and is asupercritical fluid at the temperature and pressure at which said liquidmixture is sprayed.
 8. The process of claim 6 wherein the coating powderformed has a narrow particle size distribution with a span of less thanabout 2.0.
 9. The process of claim 6 wherein the coating powder containsat least one polymer selected from the group consisting of epoxies,polyesters, acrylics, polyurethanes, epoxy-polyester hybrids, blockedisocyanates, cellulosics, vinyls, polyamides, and hybrid polymersthereof.
 10. The process of claim 6 wherein at least one gas flow isapplied to the substantially decompressive spray to increase the rate ofturbulent mixing or the temperature within the spray or both.
 11. Aprocess for forming solid particulates which comprises:(1) forming aliquid mixture in a closed system, said liquid mixture comprising: (a) awater-borne composition comprising:(i) a nonvolatile materials fractionwhich is solid or capable of becoming solid, which is capable of beingsprayed, and which is capable of forming solid particulates byevaporation when sprayed; and (ii) a solvent fraction containing atleast water; which is sufficiently volatile to render, and containswater in an amount which renders, said water-borne composition capableof forming solid particulates when sprayed; and (b) at least onecompressed fluid which is a supercritical fluid at the temperature andpressure at which said liquid mixture is sprayed and which issubstantially present in said liquid mixture as a finely dispersedliquid compressed fluid phase, in an amount which renders said liquidmixture capable of forming a substantially decompressive spray, whereinthe compressed fluid is a gas at standard conditions of 0° Celsius andone atmosphere pressure (STP); and (2) spraying said liquid mixture at atemperature above about 40° Celsius and a pressure that gives asubstantially decompressive spray by passing the mixture through anorifice into an environment suitable for forming solid particulates byevaporation.